Provisioned provider link state bridging (PLSB) with routed back-up

ABSTRACT

A method of managing traffic flow in a packet network. A working sub-network is provided, which comprises one or more provisioned static working paths between at least one source node and one or more destination nodes in the network, and the working sub-network with a service instance. A backup sub-network is provided, which comprises one or more dynamic protection paths between the at least one source node and the one or more destination nodes, and the backup sub-network associated with the service instance. During a normal operation of the network, forwarding subscriber traffic associated with the service instance through the network using the working sub-network. Following detection of a network failure affecting the service instance, the subscriber traffic associated with the service instance is switched for forwarding through the network using the backup sub-network.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the first application filed for the present invention.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to management of traffic forwarding inpacket networks, and in particular to provisioned Provider Link StateBridging (PLSB) with routed back-up.

BACKGROUND OF THE INVENTION

Network operators and carriers are deploying packet-switchedcommunications networks in place of circuit-switched networks. Inpacket-switched networks such as Internet Protocol (IP) networks, IPpackets are routed according to routing state stored at each IP routerin the network. Similarly, in Ethernet networks, Ethernet frames areforwarded according to forwarding state stored at each Ethernet switchin the network. The present invention applies to communications networksemploying any Protocol Data Unit (PDU) based network and in thisdocument, the terms “packet” and “packet-switched network”, “routing”,“frame” and “frame-based network”, “forwarding” and cognate terms areintended to cover any PDUs, communications networks using PDUs and theselective transmission of PDUs from network node to network node.

Multicast forwarding of data packets (where packets are sent from asource node to multiple destination nodes more or less simultaneously)is of increasing importance as demand for services such as PTV and Videoon Demand (VoD) grows.

In Ethernet networks, Provider Backbone Transport (PBT), also known asProvider Back-Bone Bridging-Traffic Engineering (PBB-TE), as describedin Applicant's British patent number GB 2422508 is used to provide aunicast Ethernet transport technology. Provider Link State Bridging(PLSB) as described in Applicant's co-pending U.S. patent applicationSer. No. 11/537,775 can be used to provide a multicast transportcapability for Ethernet networks using IS-IS to set up unicast paths andmulticast trees in the network. Both above patent documents are herebyincorporated by reference.

Provider Link State Bridging (PLSB) typically uses protocols such asIntermediate System-Intermediate System (IS-IS) or Open Shortest PathFirst (OSPF) to exchange topology, addressing and service information toenable the calculation of paths for forwarding packets from any givensource node to one or more destination nodes, and to install theforwarding state required to implement those paths. OSPF and IS-IS arerun in a distributed manner across nodes of the network so that eachnode will locally compute paths based on the view of network topologyshared by the routing system.

As is known in the art, IS-IS and OSPF are “routing” protocols, in which“Dijkstra” or similar algorithms are used to compute shortest pathsbetween any two nodes in the network. Once computed, these shortestpaths can then be used to derive unicast paths and multicast trees, andto determine the forwarding state that must be installed in each node inorder to implemented the derived paths and trees. Techniques such asReverse Path Forwarding Check (RPFC) can be used to mitigate the effectof any loops that may form transiently during periods when multipledistributed peer nodes independently compute paths and install theforwarding state.

An advantage of the above described system is that the same algorithmscan be used both at the time that a new service instance is requested,and to recover from a network failure. For example, when a clientrequests the provisioning of a multicast tree between a given sourcenode and a set of destination nodes, information shared via IS-IS can beused by each node to compute the multicast tree, and install theappropriate forwarding state in each node traversed by that tree. If atopology change occurs in the network, such as a node or link failure,this information will be flooded to all nodes by the protocol'soperation, and each node will locally recompute paths as required basedon a consistent view of the changed network topology.

While this approach provides a flexible method for recovering fromnetwork failures, it suffers a disadvantage in that there is no easy wayto implement traffic engineering in a network that is operating properly(i.e. in the absence of a network failure). It computes paths on thebasis of available capacity, not offered load. Optimization of paths onthe basis of offered load is a substantially more computationallyintensive problem typically requiring offline planning tools.

Techniques for managing traffic flows, especially multicast trafficflows, in a packet network which overcome at least some of theabove-noted issues remain highly desirable.

SUMMARY OF THE INVENTION

Thus, an aspect of the present invention provides a method of managingtraffic flow in a packet network. A working sub-network is provided,which comprises one or more provisioned static working paths between atleast one source node and one or more destination nodes in the network,and a service instance is associated with the working sub-network. Abackup sub-network is provided, which comprises one or more dynamicprotection paths between the at least one source node and the one ormore destination nodes, and the service instance is associated with thebackup sub-network. During a normal operation of the network, subscribertraffic associated with the service instance is forwarded through thenetwork using the working sub-network. Following detection of a networkfailure affecting the service instance, the subscriber trafficassociated with the service instance is switched and forwarded throughthe network using the backup sub-network.

The working and backup sub-networks can be virtualized on a commonphysical network infrastructure.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a block diagram schematically illustrating a Provider LinkState Bridging (PLSB) domain in which methods in accordance with thepresent invention may be implemented.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a method of managing traffic forwardingin packet networks, particularly applicable to provisioned Provider LinkState Bridging (PLSB) with routed back-up. Embodiments of the inventionare described below, by way of example only, with reference to FIG. 1

In very general terms, the present invention provides methods formanaging traffic flow in a packet network, in which different methods ofpath computation and failure recovery are implemented, based on thestate of the network. More particularly, during a normal operating stateof the network, subscriber traffic is forwarded through static pathswhich are provisioned between source and destination nodes. Thesubscriber traffic may therefore be routed along paths which best matchthe available capacity to the offered load, as determined by off-lineplanning tools. Failure recovery is accomplished by path switching atthe network edge. When a service impacting failure occurs, subscribertraffic is forwarded through dynamically computed paths, and subsequentfailure recovery is accomplished using dynamic path recomputation tocircumvent the impact of further point(s) of failure.

Ethernet bridging technology as extended via PBB-TE and PLSB inparticular facilitates this model. One can envision a Backbone VLANIdentifier (B-VID) as an identifier of a sub-network which encompasses aset of one or more paths. A service instance can be arbitrarily assignedto B-VIDs, so that subscriber traffic of the service instance can beforwarded through the sub-network(s) identified by the assignedB-VID(s). Finally, different operational behaviours can be ascribed todifferent B-VID identified sub-networks. With this arrangement, the actof path switching is generalized to a concept of “network switching”;that is, changing a service association from a provisioned “working”sub-network (B-VID) to a predetermined “backup” sub-network (B-VID),which may be either a diverse provisioned sub-network (B-VID) or adynamically routed sub-network (B-VID). Further, it is expected thatmany services may share the network connectivity associated with aB-VID, and one can envision scenarios in which a given B-VID is aworking B-VID for some services, and a backup B-VID for others.

The present invention is equally applicable to unicast paths andmulticast trees installed in the network. For convenience ofdescription, specific reference will be made to “paths”, it beingunderstood that such references shall apply equally to both unicastpaths and multicast trees.

For the purpose of understanding the present disclosure, “static” pathsshould be understood to be unicast paths and/or multicast trees that areprovisioned prior to instantiation of a subscriber traffic flow throughthat path, and remain “nailed-up” for at least the duration of asubscriber communications session or subscribed service instance. Ifdesired, the route traversed by a static path may be computed using a“Dijkstra” algorithm. Additional considerations (such as, for example,network traffic engineering objectives and Service LevelAgreements—SLAs) may also be used in the route computation. Once theroute of a static path has been computed, forwarding state implementingthe path is installed in each of the appropriate nodes, so thatsubscriber traffic can be properly forwarded through the path. It isanticipated that static paths will normally be computed “off line” (forexample by a Operations Administration and Maintenance, OAM, server)rather than by means of a distributed computation process such as IS-ISor OSPF (although the server may utilize substantially the samealgorithms). This enables the static paths to be fully computed inadvance, and using a broader set of criteria than simply shortest path.The server or other means computing the static paths will ensure that nopersistent loops are formed, and the centrally controlled “provisioning”process will ensure that there is no possibility of inadvertentlygenerating a loop during installation of the path.

Because static paths are “provisioned”, distributed path computationprocesses (e.g. IS-IS) will not attempt to modify them when a networkfailure occurs. Rather, failure recovery of a provisioned path dependson protection switching functionality at each of the involved edgenodes. For example, if a topology change occurs in the network, such asa node or link failure affecting one or more provisioned paths, thisinformation will be flooded to all nodes in the network by means ofconventional Link State Packets (LSPs). When the LSPs arrive at an edgenode (i.e. a source node or a destination node) of a service for which aprovisioned path transits the failed link (or node), a protectionswitching function is triggered for the impacted service to switch thesubscriber traffic from the provisioned path to a predetermined backuppath.

In a PLSB network, the above-noted protection switching function can beimplemented by associating a service with two B-VIDs, one of which isdesignated the working B-VID and the other the backup B-VID for thatparticular service instance. In this case, the actual switching functionis performed by changing the default B-VID for the service. Both of theinvolved B-VIDs may also be utilized by other service instances, and donot need to necessarily replicate the same working/protectionrelationship for those service instances. Although many serviceinstances may share the connectivity instantiated in a given B-VID, itis possible to envision a failure scenario that requires one serviceinstance to perform a protection switch but does not affect otherservices sharing that B-VID.

In conventional PLSB networks, the set of traffic flows for eachsubscriber is assigned a respective Service Instance Identifier (I-SID)identifying that subscriber's virtual network, and backbone edge bridgeswhich host I-components associated with a given I-SID (individual portson that virtual network) advertise that I-SID into IS-IS. In embodimentsof the present invention, this is still true, however state for a givenservice is duplicated in both the working and back-up B-VIDs. In thecase of a dynamically maintained backup B-VID, any service associatedwith that B-VID injects state into IS-IS in a normal manner, and theedge function selects which B-VID is to be used by the service instanceat any given time. With this arrangement, during normal operation of thenetwork, the working B-VID can be associated with the I-SID of thesubscriber traffic flow, so that ingress subscriber traffic arriving atan edge node can be encapsulated with the working path's B-VID, use theFDB entries for network connectivity associated with the working B-VIDand thus be properly forwarded via provisioned connectivity. If an edgenode becomes aware of a network topology change affecting any of theworking paths for the service, then the protection switch function inthe edge node would simply replace the working B-VID with the specifiedbackup B-VID. Once this switch had been made, the subscriber trafficwould (after encapsulation with the backup B-VID) be automatically usethe FDB entries associated with the backup B-VID and traffic would beforwarded via the protection paths.

Those of ordinary skill in the art will recognise that the use of apredetermined protection path, and associated protection switchingfunctionality at edge nodes is well known. However, in conventionalsystems, both the working and protection paths are provisioned insubstantially the same manner, and using substantially the samecriteria. Thus, for example, if the working path is provisioned tosatisfy a selected Quality of Service (QoS) guarantee in a Service LevelAgreement (SLA), then the corresponding protection path will normallyalso be configured to satisfy either the same QoS requirement, or someother QoS requirement as specified in the SLA. Additionally,conventional protection switching schemes are implemented at agranularity of an individual path. That is, when a network failure isdetected, subscriber traffic within a path which traverses the failednetwork element is switched to a corresponding protection (backup) path.Typically, this switching function is implemented without regard to theservices associated with the impacted subscriber traffic. This haslimitations with respect to multicast resilience, congruency of unicastand multicast, and resilience against multiple simultaneous failures.

In contrast, the present invention extends the concept of per-pathprotection switching to the granularity of sub-networks, and is aware ofimpacted services. Thus, the conventional notion of working andprotection paths is replaced by working and backup sub-networks, each ofwhich encompasses one or more paths. Furthermore, if a network failure(or other topology change) affects a service, then all of the traffic ofthat service is switched to the predetermined backup sub-network, evenif the failure only impacts one (of possibly several) paths of theworking sub-network.

In the present invention, the paths of the working sub-network (B-VID)are provisioned static paths, while the paths of the back-up sub-network(B-VID) are preferably dynamically routed using a distributed pathcomputation algorithm, such as IS-IS or OSPF, and so are maintained bythe routing system as a complete “mesh” solution at all times. With thisarrangement, the QoS of the dynamically routed backup B-VID is always“best effort”, regardless of the QoS specification of any SLA that mightapply to the provisioned working B-VID.

If desired, the protection path(s) that the network will use for a givennetwork state can be considered at the same time as the provisionedworking path(s) of the working B-VID is (are) computed. If desired, theworking path computation can be subject to one or more predeterminedconstraints. For example, the mesh of working paths may be constrainedto maximize diversity with respect to the mesh of protection paths whichwill be maintained by the routing system under fault-free conditions,which imposes physical diversity between working and protection paths.Similarly PLSB can also use equal cost path computation to achieve atleast partial but not necessarily absolute diversity of paths betweenany two points. The normal mode of operation envisioned is that thecombination of the manipulation of topology constraints and use of theranking of equal cost tie breaking for a given constraint set willproduce a set of complete provisioned mesh variations, and the set ofvariations that the dynamic routed system will produce will also bemodelled. Each is assigned a B-VID, and judicious selection of theprovisioned working B-VID and dynamic backup B-VID for a given servicewill be used to minimize the set of possible failures whichsimultaneously perturb the services on both a working B-VID and thedynamic backup VID for those services. The net result is that in manycircumstances, the dynamic backup VID is a significantly diverse paththat is continually updated to reflect the current state of survivingresources in the network but is stable and converged at the time of afailure in the working path, and as such will always restore a minimumof best effort connectivity, and by the use of per class packet markingand queuing disciplines combined with limits on network oversubscriptionwill minimize the degradation of service during an outage.

Because each protection path of the backup B-VID is dynamically computedusing a distributed path computation algorithm, that portion of theprotection mesh solution will be automatically recomputed and updatedwhen a network failure occurs that affects any protection path. Thus,whereas a provisioned working path is static, its correspondingprotection path is dynamic, in that it will be dynamically maintained inresponse to the current network state. So as noted above, when a workingpath failure occurs that does not affect the protection paths utilizedby the affected services, the result is the routing system hasmaintained the best possible fall back solution, and so switchover willbe in the order of notification and protection switching times for eachimpacted multipoint service.

As may be appreciated, this arrangement enables the planning anddeterministic benefits and rapid fail over of provisioned paths to becombined with the multiple failure tolerance of dynamic paths to berealized in the packet network. In particular, provisioned paths enabletraffic engineering, Quality of Service (QoS) guarantees and servicelevel differentiation within the network, which are difficult orimpossible to ensure with dynamically computed paths except via massiveover-provisioning of bandwidth. On the other hand, dynamically computedpaths enable a highly flexible mechanism for recovery from networkfailure, and are robust against multiple failures in the network. Forsome network service providers, these benefits will be sufficientlyimportant to justify the increased OAM complexity of maintaining twodifferent path computation algorithms, and two different failurerecovery mechanisms in a network domain.

According to the present invention, during normal operation of thenetwork, subscriber traffic is routed through a provisioned workingpath, so that the benefits of traffic engineering QoS guarantees etc.can be realized. However, if a network failure occurs, traffic for theservices impacted by the failure are switched to a protection path whichhas been computed by the dynamic routed system. The traffic that isswitched is all end points for a given service, including pathsunaffected by the failure. The concept is that the provisionedmultipoint full mesh has become a partial mesh due to failure, and henceto properly preserve all aspects of the service connectivity, inparticular the multicast aspects, all endpoints for an affected serviceare switched from the working to the protection B-VID.

Consequently, if simultaneous network failures impact both the workingpath and its protection path service restoration will be delayed untilthe dynamic path re-converges, the protection path not being stablethrough the switchover. Unlike a conventional system in which theprotection switching function will fail because the protection path isinoperative, in the present invention, the distributed path computationalgorithm (eg IS-IS or OSPF) will recompute the protection paths tocircumvent the failure(s), and so guarantee that the subscriber trafficflow can be restored.

Furthermore, it will also be seen that if, during normal operation of aworking path, a network failure were to occur that affected only thecorresponding protection path, then the distributed path computationalgorithm (eg IS-IS or OSPF) will automatically recompute the protectionpath to circumvent that fault. As a result, the protection path isalways ensured to be available in the event of a network failureaffecting the working path.

As alluded to earlier in this disclosure, it is also possible toenvision a switching hierarchy whereby the initial switchover is to analternate provisioned B-VID and a further failure such that serviceconnectivity is incomplete in all of the set of provisioned B-VIDs willdefault to a dynamic backup B-VID. However it should be noted that therequirement for a complete mesh solution for each service in each B-VIDmeans that the amount of state required in the network to achieve thisgoes up accordingly.

Similarly it is also possible to envision that some servicesinstantiated on the network do not use provisioned paths at all, andsimply depend upon the dynamic paths for resiliency. In this way aspectrum of service offerings of differing QOS and resiliency guaranteescan be accommodated on a common infrastructure.

As described above, during normal operation of the network, ingresssubscriber traffic arriving at an edge node is encapsulated with theworking B-VID, mapping all infrastructure switching for that frame tothe forwarding entries for the identified working B-VID in the FDB. Ifthe edge node becomes aware of a network failure affecting the any pathof the working B-VID, then the protection switching function in the edgenode replaces the working B-VID with the backup B-VID. Once this switchhad been made, the subscriber traffic is encapsulated with the BackupB-VID and is thus automatically forwarded through a protection pathbased on the forwarding entries for the identified backup B-VID in theFDB. In order to maintain symmetrical congruence, this protectionswitching function must execute at every endpoint of every I-SIDimpacted by the failure of the working path.

This gives rise to a requirement that some means must be provided toensure that each edge node becomes aware of all I-SIDs in a provisionedB-VID that are impacted by a network topology change, includingendpoints using connectivity disjoint from the physical failure(consider three nodes A, B, and C implementing a common service; whenthe connectivity B-C fails, in this model A, B and C all need to performa switching operation, even though the A-B and A-C connectivity mayremain intact). The local forwarding database at each node containsinformation from which an association between a given I-SID in aprovisioned B-VID and a local network link (from that node to itsimmediate neighbour) can be derived. However, this information isgenerally not available to all nodes in the network without definingadditional procedures to disseminate provisioned state into the routingsystem. As a result, when a node receives a notification of a link statechange, it is generally not possible to determine which I-SIDs in aprovisioned B-VID might have been impacted by that change, unless theimpacted link is directly adjacent an edge node.

In some embodiments those I-SIDs which traverse a given link can bedirectly encoded in the multicast addresses that point to a given linkin the FDB for the provisioned B-VID. In other embodiments the list ofI-SIDs traversing a given link can be downloaded as part of the“provisioning” process. This permits the node to have knowledge of whatprovisioned service connectivity transits a given link and be able toadvertise such information into the routing system as an attribute ofthe link. The simplest embodiment is to advertise this information onlyfor failed links (in the LSPs announcing the link failure), in order tominimize the overall amount of state in the routing database.

A node receiving LSPs indicating link failure simply constructs thelogical “OR” across the set of LSPs of the set of I-SIDs for which theworking or provisioned VID is no longer viable, determines which ofthose I-components are instantiated locally, and instructs the localimplementation to use the working or protection VIDs accordingly. Itwill be observed that such a technique is automatically revertive fortraffic for which the engineered working VID has again become available.

An advantage of this approach is that the conventional forwardingdatabase entries and link state advertisement process, which arecommonly utilised in routed packet networks are exploited to also conveyinformation needed for proper protection switching of provisioned paths.Furthermore, this is accomplished without modifying the conventionaltraffic forwarding and dynamic route computation processes, at least thelatter of which is needed for routing of the protection paths. However,this approach also suffers the possible disadvantage that, in a casewhere the provisioned paths for multiple I-SIDs traverse a link in thenetwork, LSPs advertising state changes of that link will necessarilycontain a listing of all of the affected I-SIDs. In some cases, this canrepresent an onerous amount of data.

In some embodiments, this disadvantage may be overcome by computing afixed-length digest (e.g. a bloom filter) of the I-SIDs that haveprovisioned FDB entires that traverse a given link, and advertising thatdigest in the LSPs announcing the failure of that link (instead of theexplicit list of affected I-SIDs). Each edge node can also generate asimilar digest of I-SIDs that it hosts. With this arrangement, when anLSP containing a digest is received at an edge node, the digest can beextracted and compared to the local digest to very quickly determine ifany I-SIDs hosted by the edge node need to be switched to protectionpaths and, if so, which ones. A consequence of this is that switching ofsome services unaffected by a given failure may occur, which is theconsequence of the lossy compression of knowledge of the impactedservices inherent in a fixed-length digest.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

We claim:
 1. A method of managing traffic flow in a packet network, themethod comprising: providing a working sub-network comprising one ormore provisioned static working paths between at least one source nodeand one or more destination nodes in the network, and associating aservice instance with the working sub-network; providing a dynamicbackup sub-network comprising one or more dynamically maintainedprotection paths between the at least one source node and the one ormore destination nodes, and associating the service instance with thedynamic backup sub-network; during a normal operation of the network,forwarding subscriber traffic associated with the service instancethrough the network using the working sub-network; and followingdetection of a network failure affecting the service instance, switchingthe subscriber traffic associated with the service instance forforwarding through the network using the dynamic backup sub-network. 2.The method as claimed in claim 1, wherein the working sub-networkcomprises a set of two or more static working paths provisioned betweenrespective sets of source and destination nodes, and wherein, followingdetection of a network failure, the subscriber traffic being forwardedin each one of the set of two or more static working paths is switchedfor forwarding through corresponding dynamic protection paths of thedynamic backup sub-network.
 3. The method as claimed in claim 1,wherein, during normal operation of the network, respective forwardingstate is installed in the network for both the static working paths andthe dynamic protection paths.
 4. The method as claimed in claim 3,wherein forwarding traffic using the provisioned static working pathcomprises: assigning a Backbone VLAN Identifier (B-VID) of the workingsub-network to the subscriber traffic for the service instance, andforwarding the subscriber traffic in accordance with the assigned B-VID.5. The method as claimed in claim 4, wherein switching the subscribertraffic of the service instance for forwarding using the backupsub-network comprises: assigning a B-VID of the dynamic backupsub-network to the subscriber traffic; and forwarding the subscribertraffic in accordance with the assigned B-VID.
 6. The method as claimedin claim 1, wherein providing the working sub-network comprises, foreach provisioned working path of the working sub-network, installingrespective state for the provisioned working path in a database of eachnode traversed by the provisioned working path, and wherein the statecomprises information of a provisioned service instance identifier(I-SID) of the service.
 7. The method as claimed in claim 6 wherein theinformation comprises the I-SID information encoded within a destinationaddress of the provisioned working path.
 8. The method as claimed inclaim 6 wherein the information comprises a digest of two or more I-SIDsencoded within a data element associated with the provisioned workingpath.
 9. The method as claimed in claim 8 wherein the digest is a bloomfilter.
 10. The method as claimed in claim 6 further comprising, at anode traversed by the provisioned working path: detecting a networkfailure affecting at least one service; and flooding a Link State Packet(LSP) containing the information of the provisioned service instanceidentifier (I-SID) of the at least one service.
 11. The method asclaimed in claim 10, further comprising, at an edge node of the network:receiving the LSP flooded by the node traversed by the provisionedworking path; and comparing the information contained in the receivedLSP to information of service instance identifiers (I-SIDs) of serviceshosted by the edge node, to identify any services hosted by the edgenode that are affected by the detected network failure.
 12. The methodas claimed in claim 1, further comprising providing a provisioned backupsub-network comprising one or more provisioned protection paths betweenthe at least one source node and the one or more destination nodes inthe network, and associating the service instance with the provisionedbackup sub-network; and wherein: following detection of a first networkfailure affecting the service instance, the subscriber traffic isforwarded using the provisioned backup sub-network; and followingdetection of a second network failure affecting the service instance,the subscriber traffic is forwarded using the dynamic backupsub-network.